IPv6 Essentials

Prefix Notation

The notation for prefixes has also been specified in RFC 2373. A format prefix is the high-order bits of an IP address used to identify the subnet or a specific type of address (refer to Table 3-2). In newer drafts, it is called the global routing prefix. The prefix notation is very similar to the way IPv4 addresses are written in Classless Interdomain Routing (CIDR) notation, and it is also commonly used for subnetted IPv4 addresses. The notation appends the prefix length, written as a number of bits with a slash, which leads to the following format:

IPv6 address/prefix length

The prefix length specifies how many left-most bits of the address specify the prefix. This is another way of noting a subnet mask. Remember, a subnet mask specifies the bits of the IPv4 address that belong to the network ID. The prefix is used to identify the subnet that an interface belongs to and is used by routers for forwarding. The following example explains how the prefix is interpreted. Consider the IPv6 prefix notation 2E78:DA53:12::/40. To understand this address, let’s convert the hex into binary as shown in Table 3-1.

Table 3-1.  Understanding prefix notation

Hex notation	Binary notation	Number of bits
2E 78 DA 53 12	00101110 01111000 11011010 01010011 00010010	16 bits 16 bits 8 bits, total 40 bits

The compressed notation (replacing a sequence of zeros with a double colon) is also applicable to the prefix representation. It should be used carefully, though, ...